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 Post subject: repairing a BC-923a tank receiver
PostPosted: Feb Thu 01, 2018 5:06 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
Other than a B/W pic or 2 and 1 small chapter in a manual of other radios, I haven't been able to find anything concerning this radio. Evidently it never was very popular with the ham community judging by this lack of websites devoted to it. Or maybe I just haven't used the proper google search words yet to find more about it.

Anyway, its a rusty old boatachor that was sitting on a concrete floor for years next to a leaky car battery from what I have been told. It has a well made external AC power supply. I replaced the 2 caps in it and its working fine. I decided to plug it into the radio and slowly bring it up on the variac while monitoring AC current consumption and B+ voltage. It powers up and is working. The radio started working at about 75VAC and I left it there for a while and gradually moved up to about 100VAC over the course of an hour or so. I shut it down and started cleaning switches and controls and then printed out the pages I found in a service manual. I fired it up again and listened to some Cb'ers while reading the printouts for alignment and other servicing info.

I'm still wondering how to proceed with this radio. Seeing so little about it on the web means either its not a popular or desirable radio and I can repair and mod as I wish, or that its very rare and I should do what I can to bring it back to original glory. If I keep the radio I had the bright idea of tapping off the final I.F stage, wire in a small board with a solid state detector circuit and connect it to the first audio stage while bypassing the 3 FM conversion stages. For whatever reason, there already is an oscillator tube stage running at the IF frequency. I can tap off of that and run to my detector board for BFO. That way I have AM and SSB. by the way, the radio was designed for 27-36 mhz FM. So other than the rarely heard FM stations on 10 meters I figured it would be better to mod it for CB listening. As planned in my head the mods would be minimally destructive involving mostly unplug the un-needed tubes and attach a half dozen or so wires from my detector board. I'd be happy if it turns out that easy but after a more detailed look at the chassis and schematic I will know for sure.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Thu 01, 2018 6:30 pm 
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Outside of TM 11-310 (probably the one you already have) you probably won't find too much on this one. It wouldn't have been terribly popular with hams given its tuning range and design for FM. It wasn't popular enough to make the group of 3 Surplus Conversion manuals and I don't recall ever running across an article about one. Today it would be of most interest to those trying to restore a tank :)

Rodger WQ9E


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Thu 01, 2018 6:52 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
Yeah, that TM 11-310 was the only thing I found. I guess I finally found something that doesn't have a youtube video. :lol: Here is a pic without the case. I have that out in the garage because of the acid damage and rusty odor.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Thu 01, 2018 8:29 pm 
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Location: Milwaukee,WI
If 6 meter FM was as busy as it used to be, it would be worth building a simple converter at the front end.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sat 03, 2018 5:40 am 
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Joined: Apr Thu 12, 2007 3:24 am
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Location: Milwaukee,WI
I decided on my plan of attack. I'm going to try installing a simple diode detector stage for AM reception. If all goes as planned, all I will need to do is remove the 2 limiter stage tubes from their sockets, lift one leg of a cap and resistor, then attach a ground and 2 wires from my detector board to some pins on the tube sockets. Very easy to reverse if it doesn't work out well. While probing around to discover why the rf gain control doesn't work ( its at full gain all the time ) I found a clipped wire sticking out of one of the main wiring harnesses. If it hadn't been clipped I might have had a good chance of figuring where it was supposed to connect to. But since it was cut it could have gone almost anywhere. And with a lack of these radios I seriously doubt I could find someone to trace that wire in their radio.

I suppose I should make some tube socket voltage and resistance checks before doing my mod to see if I can figure out that missing wire and rf gain issue. But I'm not real concerned about either of those because the radio is running fine otherwise.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sun 04, 2018 2:37 pm 
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Location: Milwaukee,WI
While doing a detailed look at the difficult to follow schematic I realized that the front panel control marked "sensitivity" is not RF Gain, its the Squelch adjustment to go along with the Squelch On/Off switch. However it still doesn't work. But I rarely use squelch on an AM receiver so I can live with that if I can't figure it out.

I had success with installing my simple diode detector for AM. But I did notice a few things. After alignment the receiver seems to have a little bit too wide IF bandwidth. I wonder if it was designed that way because the radio is supposed to be FM. Maybe IFT values or coil spacing was designed for wider bandwidth. If so, I'll have to live with that too. I know I could use the same IF out tap point for the detector to route the 2580khz signal to another receiver for better selectivity but that would spoil the charm of running the old rig by itself.

After connecting the detector with clip leads I was measuring various things. I found that the input of the first audio tube, where I connected the detector output, has 45 volts on it when measured from chassis ground. Luckily the coupling cap on the output of my new detector circuit kept that voltage away from the rest of the detector. Again it boils down to squelch control and how it cuts off the audio tubes.The radio works fine as is but I'm considering trying to rewire the audio half of that dual triode to a simpler, squelchless stage. It doesn't look to be easy to work on the radio so time will tell if I perform that mod or not.

Reception was great with lots of sensitivity and clear audio. But then someone a mile or so away with a strong signal started talking and they were distorted. So while they were talking I went stage by stage with my trusty old Eico signal tracer and rf probe and traced it down to the 2nd IF stage. I'll need to look into that now too.

I saw online that Ace Hardware and other places carries a flat green Rustoleum spraypaint that looked close enough in color for me. If a friend of mine still has his sand blaster I'll take the cabinet to him. If not, I'll go at it with wire brush and emery cloth. But there is no rush on that because I have no where indoors to paint. that will have to wait for Springtime. :cry:

I just had a thought. If the distortion in the 2nd IF stage is from too much gain maybe I could add avc to my detector circuit and run it to the input of that stage, the first IF stage or RF stage. Whichever one makes more sense.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sun 04, 2018 4:22 pm 
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Nice job of documenting all you have done to this receiver.

In its original incarnation the distortion developed in the RF stage wouldn't have been a concern until the overload became severe enough that it was operating as a mixer and generating spurious signals. With the fairly minor overload you experienced it was just operating as another limiter for FM but for AM use you will need to either implement some manual or automatic control of gain to prevent overload in one of the pre-detection stages.

If you decide to make a circuit change to provide selectivity more suited to AM consider doing what National did with their lowest cost members of the sliding coil catacomb family, the NC-80X and NC-81X receivers. The original member of this family was the NC-100 and probably the best known (and best performer) is the NC-2-40D. Unlike the other family members the low cost general coverage NC-80X and ham band only NC-81X are AC/DC type receivers and they don't have a RF stage and instead National uses a 1560 Khz. IF for sufficient image rejection. To provide sufficient selectivity these models have a single element crystal filter that is always in circuit and the "on the nose" selectivity is continuously variable from 7 Khz. down to 300 hz. Of course like all single element filters the shape factor is not good but it does provide a useful function. I picked up both of these receivers some time ago and although they won't replace my favorite NC-2-40D or my Pierson KP-81 which is the king of the sliding coil catacomb receivers the low cost members are very usable when conditions are reasonable.

You could implement this circuit so that it could plug in to one of the existing IF tube sockets with little or no modification to the set. Operating in that frequency range you could probably just set and forget the bandwidth of the filter. Finding a junker crystal very close to the current IF frequency shouldn't be too difficult and you could easily shift the IF frequency probably 100 Khz. without issue if needed to match up with a found crystal.

Rodger WQ9E


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sun 04, 2018 4:44 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
In my case the 2nd IF stage is producing the distortion. I haven't yet researched why. It could be a faulty component or just too much signal. Physically its much easier to get at the IF sections than it is the RF and mixer sections which are buried under another module which would have to be removed. So I was thinking of connecting the agc voltage to the 1st IF and see if it lowers the signal enough to stop the overload of the 2nd iF.

Its a 2850khz IF. I have a box of unsorted crystals but I doubt I'll find anything close enough. After getting everything else done I'll consider a crystal filter. Its not as though the 10 and 11 meter bands are super crowded these days.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sun 04, 2018 7:38 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
A little more progress made on the radio. I rewired the first audio tube to get rid of any involvement with the squelch circuitry. It now is a standard triode configuration with a cathode resistor and cap to ground and a signal grid resistor to ground. Then the output of my detector board connected to the grid. It sounds slightly fuller in response and gain is very close to the same.

I tapped off of the detector board with a 1 meg resistor and connected to the inputs of first the 1st then 2nd IF stage. It made a difference on all but the strongest signal from my generator. I don't have a chart handy but I don't think any normal over the air signal will reach that amount. I'm not quite sure yet on which stage I will connect the agc voltage to and with what value resistor. With so many things clip leaded in I started to get a little instability. So I will hard wire a few connections and give it another try next time.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Mon 05, 2018 2:22 am 
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Location: Milwaukee,WI
Hey, Surplus Sales of Nebraska shows 2850khz crystals. What a stroke of good luck.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Mon 05, 2018 4:52 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
I spent hours doing web searches because I was determined to find out more about this radio. I ran across an interesting bit of info on a Russian website that lead me to find a few more sites. My BC-923A receiver was part of a setup designated SCR-808 and SCR-828 which included a transmitter with either 2 or 1 receiver. Searching those brought a few more sites. But it still is a rare radio. One guy in a forum was saying in his 40 years of the Military radio hobby he had never seen the matching transmitter and only 1 receiver. Another article in a different site mentioned also how rare it is. Because it looked so much like a BC-603 style tank radio I assumed mine was too. But there it was inside a jeep. It says these were used by Coast Artillery batteries and Military Police units. I also found out that not only is it an FM receiver but its a wideband FM receiver. The docs mention that even though the receiver and transmitter are variable, not crystal controlled, they use 100kc spacing for use. That doesn't mean the bandwidth is 100kc. I'm sure they must have made allowances for a gap in between channels and that the actual receiver bandwidth is somewhat less. I'm curious enough to measure that now. I'll post the results. If it is indeed very wide that homebrew crystal filter idea is looking more inviting.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Mon 05, 2018 7:48 pm 
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Joined: Apr Thu 12, 2007 3:24 am
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Location: Milwaukee,WI
I injected the equivalent to an S9 signal into the radio. AM, 60 percent modulation, 1khz tone. I adjusted the signal generator frequency until I could no longer hear the signal. That was roughly 35khz to either side. So unscientifically speaking the receiver has roughly a 70khz bandwidth. That sucks. I gave a quick look at how to build and install a filter. Building the filter is easy. Figuring out input and output impedances and matching them would be the tougher part.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sat 10, 2018 5:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3432
Location: Monterey California USA
If this receiver uses a circuit similar to the BC 603, there is a choke in the cathode of a 6AC7 which is used as a limiter, and you can receive AM by turning the limiter into an infinite impedance AM detector and a toggle switch then will select AM or FM. This is described in the Surplus Conversion Manual Vol. III for the BC-603 (683) at p. 19. I just don't recall if the 923 uses that choke at the limiter cathode. If not you could add one.

Regarding the excessive bandwidth. Yes, this is the aggravation with all this series of set, including BC 603 and BC 683. The entire 10 Meter FM allocation on the amateur band comes in at once and it's just one big heterodyne. I have not done this yet but in looking at the schematic there appear to be swamping resistors across the secondaries of the IF transformers (referred to as FL1 through FL3.) Removing them may narrow things up quite a bit. An issue with narrowing up some surplus equipment like this is that you then discover oscillator design deficiencies that were masked by the wide bandwidth.

Another issue with these is that the image rejection is very poor, due partly to the choice of IF frequency. I used this to advantage to listen to the highway patrol on 42 MHz on my BC-683, where they came in at 38 MHz on the dial (the 683 does not tune up to 42.)

I assume this is just a hobby project to see if it can be done, because you can get a $ 10 programmable scanner at a hamfest that will vastly outperform this on FM. And as far as AM, a $ 10 flea market PLL synthesized CB with the jumpers at the PLL chip changed will also outperform this receiver on AM and even give you a transmitter as well.

EDIT: I see the 923 schematic is in Caringella and Clark's Surplus Handbook at Page 52. It is quite a bit different from the BC-683 and does not have the choke nor the swamping resistors, and uses two stages of limiting. More work needed on that to get AM and narrower bandwidth!

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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Sat 10, 2018 10:43 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1342
Location: Milwaukee,WI
I'm also limited by what stages of the radio I can physically get to for modifying. Some tube sockets and transformer pins are just plain unreachable. I can access the 1st IFT with a little difficulty for some pins, the 2nd IFT is out in the open, the 3rd IFT is unreachable but I was able to tap the output to my simple diode detector board from a component connected to that trans at the 1st limiter tube. Were I top clip out a bunch of components first, I could get to at least 1 of the 2 limiter tubes for rewiring. But that is a big task and I would prefer to build something outboard like I did with my detector board.

Another thought I had was using an IFT I wound for my ARC-5 radio before I found the original at Fair Radio Sales. It was for 2830khz. Only 20 khz lower that for the 2850khz needed for the 923. I made it with adjustable slugs so I know I can shift it over. I know its overcoupled which might give me a tighter bandwidth. I'm guessing I should try replacing the 1st IFT with the homebrew because it would do the most good there as opposed to further down the chain.

And as I mentioned in a previous post, I found a cheap source for a pair of 2850khz crystals if I wanted to try the crystal filter route. From reading it sounds like they are pickier with input and output matching and I could end up with "humps" for a frequency response.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Mon 12, 2018 12:35 am 
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Location: Milwaukee,WI
I've had this radio on the bench for too long and I have more radios waiting. So I got rid of all the jumper wires from testing and soldered everything up. I put the radio back in the case after giving it a rubdown with a heavy concentration of baking soda in water to hopefully stem the tide of battery acid corrosion on the lower case and chassis. I wirebrushed off a lot when I first brought the radio home but it still needed some cleanup. I will continue work on modding the chassis for my purposes after I sand blast and paint the radio in the spring. Until then I have to set it aside somewhere in the basement because its still too yucky to bring into my radio room.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Wed 14, 2018 6:34 pm 
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Concerning the AGC, you really do need to add it to the RF amp stage if at all possible.

For the xtal filter if you desire a variable bandwidth, why not repurpose the squelch control to serve as the variable bandwidth control or if you can find a rotary switch small enough perhaps you could make an xtal filter with a few fixed bandwidths.

Perhaps one of the limiter tube sockets could be repurposed for the xtal filter.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Wed 14, 2018 11:46 pm 
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Before I packed it back up I wired the squelch control as rf gain. The value was far from ideal so I hacked it with a fixed resistor bridged over it and all the control is with maybe the top 15 percent of travel. The reason is I don't have a set of tiny Bristol drivers needed to remove the setscrew in the knob. Its too crudded up to get out with a jeweler's screwdriver. When I get back to the radio later on maybe I'll just drill it out and retap for a larger set screw.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Thu 15, 2018 3:34 am 
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The bristol drivers are a must to have especially if you ever get a Collins R-390 or any other receiver that uses them.

As I recall the set I got wasn't that expensive either.

RF gain is nice.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Feb Tue 27, 2018 9:12 pm 
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Location: Milwaukee,WI
I really wish I would have taken pics from when I first brought this radio home and started with the wirebrushing and first stage of cleaning with it on my truck's tailgate before I could even take to the basement workshop. It was massively rusted, corroded from battery acid because of being stored on the ground next to a leaking car battery, scratched and dirty. I didn't take pics because I had my doubts if I was going to try and save this poor radio.

Since we somehow managed to have a 60 degree day in Feb I decided to start the Spring paint job a little early. I did some more wirebrushing and then painted. I used Rustoleum spray paint call Camouflage. It turned out to be a bit lighter than original but I'm not a perfectionist by any means. I still think its a night and day difference from when I got the radio. I did the case and face trim ring but still need to clean and mask off the chassis to finish the job. You can see that part in between the bright new green paint. Overall I'm thrilled with the new look even though some purists may gasp at the blasphemy. Looking at the unpainted part and the rust down in the bottom left corner and handle, you get an idea of how bad the radio was.


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 Post subject: Re: repairing a BC-923a tank receiver
PostPosted: Apr Mon 30, 2018 3:40 am 
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Location: Milwaukee,WI
Today actually felt warm enough for a spring day so I went outside and finished painting the radio. Then I went back inside and stripped down the power supply and painted the chassis, choke and power transformer to match the radio.

Guys at the swapfest where this came from are hardly going to recognize this again. It was a dirty,cruddy,rusty,acid corroded stinky wet radio when it left the building. I'll be sure to post some pics when its all done. I'll finish up the rewiring of the power supply tomorrow and snap a pic or 2 of that. Again I wish I would have taken a pic of the radio and supply when I was taking it off the back of my truck. The pics earlier in this post were after it already had phase one of the cleaning process. I had wirebrushed off a lot of rust and tried to rinse off as much of the acid corroded areas as I could.


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